Use Travis CI built in pip cache support

jdufresne · sigmavirus24 · commit 2ed99483f189 · 2017-08-07T19:41:59.000-05:00
https://docs.travis-ci.com/user/caching/#pip-cache
diff --git a/.travis.yml b/.travis.yml
@@ -2,9 +2,7 @@ language: python
 
 sudo: false
 
-cache:
-    directories:
-        - $HOME/.cache/pip
+cache: pip
 
 matrix:
   include:
